MP2618 – 2A, 24V INPUT, 600kHz 2-3CELL SWITCHING LI-ION BATTERY CHARGER

MP2618 Rev. 0.92

www.MonolithicPower.com

17

7/14/2010

MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.

© 2010 MPS. All Rights Reserved.

APPLICATION INFORMATION

Setting the Charge Current

1. Standalone Switching Charger

The charge current of MP2618 is set by the

sense

resistor

RS1.

The

charge

current

programmable formula is as following:

()

()

mΩ

RS1

200mV

A

I

CHG

=

(1)

2.

Switching

Charger

with

Power

Path

Management

Figure 7 shows the charge current sharing with

the system current.

Figure 7— Charge current sharing with

System current

The gain of the system current is set as:

RG1

RGS1

Gain

=

(2)

from:

RG1

RGS1

RS2

I

Gain

RS2

I

V

SYS

SYS

OUT1

×

×

=

×

×

=

(3)

accordingly, the relationship should be:

(4)

RG1

RGS1

RS2

RS1=

(5)

RGS1/2 causes the charge current sense error

as it changes the sense gain of A2, which can be

calculated from:

()

()

()

kΩ

RGS

kΩ

2

kΩ

12.3

G

A2

+

=

(6)

The charge current is set as:

()

()

mΩ

RS1

G

1230

A

I

A2

CHG

×

=

(7)

is:

()

()

()

mΩ

RS1

10

Ω

RGS

2000

A

I

CHG

×

+

=

(8)

To decrease the power loss of the sensing circuit,

choose RS2 as small as possible, 20m is

current sense error of the system current, 50Ω is

at least.

Substitute these two values into equation (5),

then the calibrated charge current set formula in

power path application is got from equation (8):

()

()

()

mΩ

RS1

10

mΩ

RS1

2.5

2000

A

I

CHG

×

×

+

=

(9)

Following table is the calculated RS1 and RGS1

value for setting different charge current.

RGS(Ω)

RS1(mΩ)

2

280

110

1.5

402

160

1

665

260

0.8

909

360

0.5

2k

800

If choose different RS2 and RG1, re-calculated

from equation (5) and equation (8), then get the

different equation (9) and the table.

can be realized by re-calculate equation (4), (5)

and (8).

RG1

RS1

RGS1

RS2

∆I

RS1

∆V

∆I

SYS

OUT1

BAT

×

×

×

=

=